FIG. 10 shows the configuration of a voltage conversion circuit using a conventional charge pump circuit. In this voltage conversion circuit, the charge pump circuit comprises capacitors c1, c3, switches s1, s2, s3, s4, oscillator 100 and inverter 102. Among switches s1, s2, s3, s4, switches s1, s3 are set ON/OFF simultaneously by pulse signal pa from oscillator 100 as the switching control signal, while switches s2, s4 are set ON/OFF simultaneously by pulse signal pb from inverter 102. As pulse signal pa output from oscillator 100 and pulse signal pb output from inverter 102 are opposite in phase, when s1, s3 are turned ON, s2, s4 are turned OFF, and when s1, s3 are turned OFF, s2, s4 are turned ON.
When switches s1, s3 are ON and switches s2, s4 are OFF, capacitor c1 is charged to voltage V.sub.CC by a power source with an output voltage of V.sub.CC through switches s1, s3. Then, as switches s1, s3 are turned OFF and switches s2, s4 are turned ON, capacitor c3 is charged to V.sub.CC by the voltage on capacitor c1 through switches s2, s4. As power source voltage V.sub.CC is applied to one electrode of capacitor c3, capacitor c3 is charged to V.sub.CC, and its electrode on the + side is boosted to a potential of 2 V.sub.CC. In this way, as the two groups of switches s1,s2, s3,s4 are repeatedly turned ON/OFF alternately and complementarily, a doubled voltage 2 V.sub.CC, twice the power source voltage V.sub.CC, is obtained at output terminal 104.
Capacitors c2, c4 and switches s5, s6, s7, s8 set in the latter section of the voltage conversion circuit form a polarity inverter for inverting the polarity of voltage 2 V.sub.CC at output terminal 104. Among switches s5, s6, s7, s8, switches s5, s7 are turned ON/OFF together with the aforementioned switches s2, s4, while switches s6, s8 are turned ON/OFF together with said switches s1, s3.
In this polarity inverter, when switches s5, s7 are turned ON and switches s6, s8 are turned OFF, capacitor c2 is charged to 2 V.sub.CC by voltage 2 V.sub.CC at output terminal 104 or on the + side of the electrode of capacitor c3 via switches s5, s7. Then, as switches s5, s7 are turned OFF and switches s6, s8 are turned OFF, capacitor c4 is charged to voltage of 2 V.sub.CC by the voltage on capacitor c2 via switches s6, s8. Since the electrode on the + side of capacitor c4 is connected to ground potential (zero volts), the potential on one electrode of capacitor c4 becomes -2 V.sub.CC ; at output terminal 106, the output voltage 2 V.sub.CC of output terminal 104 has its polarity inverted to form an output voltage -2 V.sub.CC.
FIG. 11 shows the circuit configuration of a conventional line driver/receiver IC 110 as an example of the method used when the aforementioned voltage conversion circuit is utilized. In this line driver/receiver IC circuit configuration, voltage conversion circuit 112 is used to obtain the operating voltage of line driver 114 according to code EIA-232-D by means of a single power source voltage V.sub.CC. According to code EIA-232-D, the output voltage V.sub.0 of the line driver is in the range of +5 V&lt;V.sub.0 &lt;+15 V, -5 V&gt;V.sub.0 &gt;-15 V. In conventional voltage conversion circuit 112, since a bipolar voltage +2 V.sub.CC, -2 V.sub.CC twice the power source voltage V.sub.CC is generated, where a +5 V single power source voltage V.sub.CC is used, it is possible to obtain the drive line output voltages V.sub.DD, V.sub.SS of +10 V and -10 V, respectively, to meet the demand by code EIA-232-D.
In addition, +5 V single power source voltage V.sub.CC is only supplied as the operation voltage to line receiver 116.
On the other hand, in the portable information processing equipment recently developed, in order to realize low power consumption, the power source voltage is changed from 5 V to 3.3 V. In this case, in line driver/receiver IC 110 shown in FIG. 11, since the power source voltage V.sub.CC is input as a voltage of 3.3 V, the line driver output voltages V.sub.DD, V.sub.SS obtained from voltage conversion circuit 112 are at most +6.6 V and -6.6 V, respectively. This voltage level, however, fails to provide a sufficient margin for the EIA-232-D code.
If the conventional charge pump method is to be used to solve this problem, the only way to solve this problem is to use the voltage conversion circuit shown in FIG. 10 in which another stage of the charge pump circuit is added to form a 2-stage type charge pump circuit. In this case, the numbers of parts of capacitors c1, c3 and switches s1, s2, s3, s4 are doubled to 4 and 8, respectively. However, as shown in FIG. 11, capacitors c1, c3 are parts annexed to line driver/receiver IC 110. Consequently, as the number of capacitors is doubled, not only is the reliability of operation degraded, it also becomes difficult to form a small-sized lightweight circuit substrate. Also, as the number of switches s1, s2, s3, s4 is doubled, the IC design becomes more complex, and the cost increases.